Rotational Excitation by Slow Electrons. II

Abstract

Theoretical cross sections for the rotational excitation of homonuclear molecules by slow electrons are applied to calculation of the fractional energy loss per collision ($\ensuremath{\lambda}$) in ${\mathrm{H}}_{2}$. The theoretical losses are not more than $2.5(\frac{2m}{M})$ and except at the lowest energies studied (\ensuremath{\sim}0.1 ev) are smaller than observed.It would be desirable to have more direct experimental evidence of rotational excitation. For this reason we have calculated $\ensuremath{\lambda}$ at 77\ifmmode^\circ\else\textdegree\fi{}K in pure para-hydrogen and in normal hydrogen at that temperature. At electron energies \ensuremath{\sim}0.075 ev, the two $\ensuremath{\lambda}'\mathrm{s}$ should differ by about 50 percent. Similarly, because of the altered rotational distribution, $\ensuremath{\lambda}$ for deuterium differs from $\ensuremath{\lambda}$ for ${\mathrm{H}}_{2}$. Such differences, if observed, could hardly be accounted for on any other basis than rotational excitation.